The present invention relates to a process for synthesizing hydrocarbons from industrial waste gas streams such as acetylene off-gases.
Fischer-Tropsch (FT) synthesis of hydrocarbons has been used as a commercial process since the 1950""s. In a variety of guises, it is used to convert H2 and CO gas to hydrocarbons. Typically the products from FT synthesis are standard temperature and pressure (STP) liquids. Gases and waxes are also produced. The history of FT synthesis is elucidated in the following works:
xe2x80x9cState of the Art in GTL Technology, Report issued by Joe Verghese, Vice President Technology Oil and Gas, ABB Lumus Global, 1998;
Fischer-Tropsch Technology, report issued by the investment firm Howard, Weil, Labouisse, Friedrichs of New Orleans, author Arthur W. Tower II, Dec. 18, 1998;
Fischer-Tropsch Synthesis in the Liquid Phase, Kolbel et. al, Catal. Rev. Sci. Eng. 21(2), 225-274 (1980);
Fischer-Tropsch synthesis in the slurry phase, Schlesinger, M. D. et al., Industrial and Engineering Chemistry, 43(6) 1951 pp. 1474-79.
These works and the references contained therein are incorporated by reference in their entirety herein.
Fischer-Tropsch (FT) synthesis converts hydrogen and carbon monoxide into a wide boiling-point range of hydrocarbons. The hydrogen and carbon monoxide (synthesis gas) can be produced from a variety of carbon-bearing feedstocks and the resulting high-quality crude oil can be further processed to specific boiling-point fractions. Of special interest is the diesel fuel fraction because it requires little processing from the FT crude oil and it has desirable characteristics including very low sulfur and aromatic content, high cetane index, and it burns exceptionally cleanly in a compression-ignition engine.
FT technology was invented in Germany in the 1920""s and supplied that country with its liquid fuels during World War II. Since that time, interest in the technology has come and gone, generally in phase with increases in the cost of crude oil or supply restrictions. Two of the first plants in the U.S. were the Carthage Hydrocol Plant in Brownsville, Tex. in the late 1940""s (See Keith xe2x80x9cGasoline from Natural Gas,xe2x80x9d Oil and Gas Journal, Jun. 15, 1946, pp. 104-111) and the U.S. Bureau of Mines plant in Louisiana, Tex. in the early 1950""s (See Linz, xe2x80x9cSynthesis Test,xe2x80x9d Oil and Gas Journal, Aug. 31, 1950, pp. 42-43, and Kastens, et al., xe2x80x9cAn American Fischer-Tropsh Plant,xe2x80x9d Ind. and Engr. Chem., March 1952, pp. 450-466).
A recent peak in FT interest appears to have been stimulated by several factors including environmental issues and the resulting interest in clean-burning liquid fuels, a desire for fuels derived from secure domestic feedstocks, interest in exploiting stranded or associated gas resources and heavy oil residues, among others. The 1998 investor research report by Howard, Weil, Labouisse, Friedrich of New Orleans covers the history and background of FT as well as current efforts in the field.
Generally, conventional FT synthesis employs a hydrocarbon feed stock having an undesirable characteristic to make synthesis gas containing CO and H2 which is then passed over a Fischer-Tropsch catalyst which forms hydrocarbons with more desirable characteristics. Thus, for example, coal, which is unsuitable in its mined state for use as a motor fuel, can be converted into synthesis gas by oxidizing it under controlled conditions in the presence of water. This produces synthesis gas (primarily CO and H2), which is then used to produce hydrocarbons which are liquids under STP conditions and thus may readily be employed as motor fuel. In this manner the coal with its undesirable physical characteristics is converted into gasoline, kerosene, and diesel fuel, hydrocarbons having more desirable characteristics for engine fuels than the coal from which they were synthesized. But Fischer-Tropsch synthesis can normally only be carried out with feed streams that are carefully optimized for the process. Also, conversion of a hydrocarbon feedstock to synthesis gas and back to hydrocarbons does not have the thermal efficiency required to make it economically viable unless the FT catalysts have high carbon conversion ratios. Additional inefficiencies are inherent in the energy waste of synthesis gas production.
The present invention notes that there are certain industrial processes which produce off gases that are particularly suited to conversion to liquid hydrocarbons, using processes specified in U.S. Pat. Nos. 5,763,716, 5,645,613, 5,543,437, 5,506,272, and 5,324,335, all of which are incorporated by reference in their entirety herein. All these patents are assigned to Rentech, Inc., as is the present application.
A major portion of a typical FT synthesis plant is dedicated to the necessary conversion of hydrocarbon feedstocks to xe2x80x9csynthesis gasxe2x80x9d upon which the FT catalyst may operate. In this first step, the feedstock is then converted to a mixture of hydrogen and CO. This often requires extensive treatment to adjust the synthesis gas stream to a composition compatible with the requirements of the catalyst and operating conditions employed. This step may be eliminated by employing the waste gas streams from industrial processes which have compositions amenable to Fischer-Tropsch reactions. Such a scheme provides for improved efficiency in feedstock utilization since it eliminates the synthesis gas production step and adds value to a gas stream which would otherwise be discarded as a waste byproduct of the particular process being carried out. Typically, the apparatus for synthesis gas preparation amounts to about two thirds of the cost of a Fischer-Tropsch plant. Utilization of waste gas streams often requires some pre-Fischer Tropsch reactor processing to render the waste gas usable in FT synthesis, but at a fraction of the cost of the synthesis gas preparation section of a typical FT plant.
The FT synthesis scheme of the present invention utilizes FT technology which can accept a wide range of variable conditions such as are disclosed in the above patents and U.S. Pat. Nos. 5,621,155 and 5,620,670, which are totally incorporated by reference herein.
This FT synthesis technology can be used in conjunction with waste gas streams produced in the production of acetylene by quenching a partially oxidized natural gas stream, such as that described in U.S. Pat. No. 5,824,834, which along with the references contained therein is totally incorporated by reference herein. Applicants are presently unaware of any planned or operating units subjecting AOG or related tail gases to FT reactions producing hydrocarbons.
One aspect of the present invention is the utilization of industrial waste gas streams containing hydrogen and CO by converting what amounts to a crude synthesis gas to valuable liquid hydrocarbons.
A specific aspect of the present invention is a process for employing the waste gas of an acetylene plant to produce liquid hydrocarbon stocks which can be employed as motor fuels, thus adding value.
Another aspect of the present invention is a process for optimizing carbon utilization of the carbon content of a acetylene production plant waste gas stream in the production of commercially useful hydrocarbons, including chemical feedstocks and waxes.
Yet another aspect of the present invention is to efficiently and cost-effectively utilize the waste gas stream of an acetylene production plant in Fischer-Tropsch synthesis.
In accordance with the present invention, a process for converting industrial waste gases to liquid hydrocarbons is provided, comprising steps of:
a) collecting waste gases comprising hydrogen and CO;
b) optionally, compressing or expanding the collected waste gases to a pressure suitable for Fischer-Tropsch reactions;
c) pretreating the waste gases to produce a feed gas composition and proportions suitable for Fischer-Tropsch reactions;
d) passing the compressed feed gases through at least one Fischer-Tropsch synthesis reactor containing a FT catalyst under reaction conditions suitable for the production of hydrocarbons; and
e) collecting hydrocarbons from the products formed.
In the processes of the invention, the pretreatment of the waste gases can be employed to adjust the molar hydrogen/CO ratio in the feed gases provided to the FT reactor, and/or to remove at least a portion of at least one of the gases oxygen, acetylene, olefins and sulfurous gases. The feed gas should comprise a maximum of 20 mole percent carbon dioxide, and the molar hydrogen/CO ratio can range from about 0.5 to about 3.5.
A particularly suitable industrial waste gas is an acetylene off-gas, comprising from about 40 to about 80 mole percent hydrogen and from about 15 to about 50 mole percent CO. The molar hydrogen/CO ratio of the gas is preferably from about 0.7 to about 3.
The pressure of the feed gas can be at least 200 psia, preferably from about 200 to about 450 or 500 psia, and most preferably from about 250 to about 400 psia. With iron-based catalysts, a preferred range is from about 300 to about 400 psia.
The catalyst used in the FT reactor can be based upon cobalt, ruthenium or iron, or combinations thereof, but is preferably an iron-based catalyst. When using an iron-based catalyst, the molar hydrogen/CO ratio of the FT reactor feed can be in the range of from about 0.6 to about 3.5, preferably from about 1.2 to about 1.8, while catalysts using cobalt or ruthenium permit a range of from about 1.8 to about 2.2.
When multiple FT reactors are used, they can be connected in series and/or parallel to suit the available feed gases and catalysts; with cobalt catalysts, series reactors may serve to increase carbon conversion.
As part of pretreatment, the compressed or expanded waste gases can be hydrogenated or hydrotreated before entering the FT reactor. The gases can also be subjected to conventional water-gas shift reactions to adjust the molar H2/CO ratio. The FT reactor tail gases can be utilized in several ways to improve efficiency, including recycling at least a portion to the FT reactor, preferably via a compression step; separating hydrogen for feed in hydrogenation and product upgrading steps such as hydrotreating, hydrocracking, olefin saturation and pre-reforming.